The present invention relates to a heat-shrinkable laminate film having a center layer of a copolymer of vinylidene chloride (hereinafter referred to as PVDC), both two outermost layers of a mixture of a copolymer of ethylene and alpha-olefin of a crystalline melting point of 110.degree. to 125.degree. C. and a copolymer of ethylene and vinyl acetate (hereinafter referred to as EVA) of a crystalline melting point of 85.degree. to 103.degree. C. and two adhesive layers each of which is inserted between the center layer and the outermost layer.
Heat-shrink packaging is industrially most convenient as packing for foodstuffs having irregular shapes such as raw meats, cooked meats, cheeses and other fatty foodstuffs. In addition, since a long period of preservation is required to these packed foodstuffs, gas-barrier property is necessary for such a packaging material. Moreover, since it is necessary to subject the packed foodstuffs to heat-treatment for a certain time period for sterilization in the case where the foodstuff is cooked meat (for instance, for at least 8 minutes at 90.degree. C. against Escherichia coli), oil-resistance and peel-resistant of the sealed parts in the heat shrinkage (hereinafter referred to as peel-resistant) are required to the packing material. More in detail, there are problems in packing and sterilizing the fatty foodstuffs of breaking the thinly stretched outermost layer which is softened by fats and heat and of breaking the seal part or its vicinity of the packing material due to the heat-shrinking stress generating at the time of thermal sterilization or at the time of packing, and accordingly, a packaging material (film) of excellent oil-resistance and peel-resistant is keenly demanded in such a case.
As a heat-shrinkable film of gas-barrier property, a film consisting solely of PVDC, a laminate film of construction of EVA/PVDC/EVA disclosed in Canadian Pat. No. 982,923, and a cylindrically formed laminate film of construction of EVA/PVDC/irradiated EVA by radioactive rays disclosed in Japanese Patent Application Laying Open No. 34565/72, etc. are enumerable.
However, the film consisting solely of PVDC has defects of causing hygienically undesirable cases where the additives such as plasticizers and stabilizers in the film migrate into the packed foodstuff according to the kinds of the packed foodstuffs and becoming an undesirable state for preserving the foodstuffs due to the reduction of gas-barrier property owing to the relatively large amount of the additives for maintaining cold-resistant strength of the film, etc.
Since the laminate film of the construction of EVA/PVDC/EVA has the EVA layer as the outermost layer which is high in cold-resistant strength, it is not necessary to add a large amount of the additive into PVDC and accordingly, the problem of reduction of gas-barrier property has been solved, however, EVA has a defect of being poor in oil-resistance and peel-resistant.
In Japanese Patent Application Laying Open No. 34565/72, a cross-linked EVA by radioactive rays is used for the purpose of improving the oil-resistance, however, only as one of the outermost layers for the fear of causing decomposition of PVDC in the center layer by the radioactive irradiation. In addition, heat-resistant seal ability and peel resistance are not improved by the cross-linking by radioactive irradiation on EVA.
In order to obtain the laminate film excellent in oil-resistance, it is desirable to use a polymer of alpha-olefin of, for instance, a crystalline melting point of higher than 110.degree. C. as the two outermost layers. However, since the polymer of alpha-olefin of the crystalline melting point of higher than 110.degree. C. is poor in stretchability, it is impossible to uniformly stretch after being laminated with a PVDC layer, and accordingly, heat-shrinkable laminate film can not be available.
In addition, in cases where polyethylene of low density of a crystalline melting point of lower than 110.degree. C. or an ionomer is used, there are the defects of poor heat-resistant seal ability and pee resistance.
According to the above-mentioned reasons, a heat-shrinkable film excellent in oil-resistance, peel-resistant and gas-barrier property has been demanded in the field of food-packaging.
As a result of elaborated studies, the present inventors have found that the laminate film comprising a center layer of a copolymer (I) of vinylidene chloride, both the two outermost layers of a mixture of 20 to 70% by weight of a copolymer (II) of ethylene and alpha-olefin of a density of 0.900 to 0.950 and of a crystalline melting point of 110.degree. to 125.degree. and 80 to 30% by weight of a copolymer(III) of ethylene and vinyl acetate of a crystalline melting point of 85.degree. to 103.degree. C. and adhesive layer between the center layer and the outermost layer and having a percentage of heat-shrinkage of larger than 15% at 90.degree. C., with the proviso that the content of the copolymer(II) in the total weight of both the two outermost layers is less than 65%, is excellent in oil-resistance, peel-resistant and gas-barrier property.
It is an object of the present invention to provide a new heat-shrinkable laminate film for packaging foodstuff. It is another object of the present invention to provide a new heat-shrinkable laminate film, having an excellent properties in oil-resistance, peel-resistance and gas barriering properties.